It is known that security papers may be rendered less susceptible to counterfeiting by using invisible, machine-detectable, patterned coatings on the surface of the papers or by including security strips at least partially within the body of the papers. Security strips or threads, as they are commonly referred to, are typically introduced during the manufacture of such security papers and generally take the form of a continuous thread or ribbon of polyester, regenerated cellulose, polyvinyl chloride, or other plastics film coated with a layer of metal and/or magnetic material. In particular, the thread may take the form of: a fully metallized thread, which is presently in wide use in security documents around the world; partially demetallized threads that display positive image metal characters or indicia, currently used in United States Currency; or partially demetallized threads that display negative image or clear characters or indicia that are defined by metal boundaries, currently used in currencies such as the German Deutsche Mark. Security papers employing such partially demetallized threads are described in European Patent No. 0 279 880 while security papers employing partially demetallized threads displaying clear characters are described in U.S. Pat. No. 4,943,093. In addition to the above, the thread may take the form of: a thread coated with a coded pattern of magnetic material and with a layer of either a luminescent or a non-magnetic metal material, as described in U.S. Pat. No. 4,183,989; or may take the form of a thread employing two visible, co-extensive security detection features--namely, a machine-readable repeating pattern and metal-formed indicia, as described in pending U.S. patent application Ser. No. 08/222,657.
Threaded security papers are routinely examined for authenticity by members of the public and verified for authenticity by a variety of devices that include capacitive thread detectors, microwave detectors, eddy current detectors, x-ray detectors (e.g., a scintillation counter) and detectors that depend upon intrinsic magnetic properties such as permeability, retentivity, hysterisis loss and coercivity.
Fully metallized threads, either fully or partially embedded in security papers, are relatively easy to detect by capacitive thread detectors. However, these detectors merely detect the presence or absence of such threads and are easily fooled by lines of conductive material (i.e. pencil lines) on the surface of the document. Moreover, such threads, even when fully embedded in a security paper are visible under reflective illumination. Therefore, a pencil line drawn on the surface of a counterfeit note could easily deceive members of the public into thinking that the document is authentic.
Partially demetallized threads, such as those used in United States Currency, employ a security feature (i.e. metal characters) that can be visually detected only under transmitted illumination and that can be machine detected. However, commercially available thread detectors merely detect the presence or absence of the conductive features or characters on these threads. Due to the small size of the characters, machine reading (i.e., denomination determination) of characters or indicia is extremely difficult. Optical character recognition or other imaging based schemes would have to be employed to ascertain such detailed information.
Partially demetallized threads, such as those used in the German Deutsche Mark, employ a security feature (i.e., clear characters defined by metal boundaries) that can also be visually and machine detected. Such threads have a continuous metal path that extends the entire length of the thread which reportedly makes these threads easier to detect by commercially available thread detectors. However, only the presence or absence of these threads are detected by such detectors. Moreover, it appears that once these documents are in circulation the ability of such detectors to accurately detect the presence of the thread diminishes. This is reportedly due to the presence of cracks or voids present in the continuous metal path that result from handling of the documents. In addition, machine reading such threads would be even more difficult than machine reading the metal characters employed on the United States Currency threads where the detectable metal material merely forms the boundary of the indicia.
Threads coated with a layer of magnetic material and with either a luminescent or a non-magnetic metal material, where the magnetic material is possibly applied in a coded pattern (e.g., magnetic coating applied discontinuously onto a thread with the discontinuities detected with a field detecting device or two different magnetic materials provided in alternating bands along the thread), as described in U.S. Pat. No. 4,183,989, are machine readable but do not offer a public security feature, such as text. Moreover, relying upon the field produced by a certain magnitude or configuration of magnetic materials is problematic in that such coded variations are subject to obliteration by intentional or accidental demagnetization subsequent to the original magnetization. In addition, although magnetic material, such as iron oxide coatings, can be applied discontinuously onto a thread, in a bar code like sequence or in varying depths of coating, to accomplish a machine-readable feature, such application processes require specialty screen printing equipment to apply the iron oxide slurry in defined bars. Moreover, magnetic field array detectors are required to resolve the coded sequence. These array detectors are expensive to manufacture and are particularly problematic for reading threads when banknotes or other documents are processed narrow-edge versus wide-edge where the number of sites on the array that are processed for the wide-edge feed condition are reduced.
U.S. patent application Ser. No. 08/222,657 for "Security Threads Having At Least Two Security Detection Features And Security Papers Employing Same", filed Apr. 4, 1994, discloses a security thread employing two visible and co-extensive security detection features. A first security detection feature comprises a machine-readable repeating pattern made up of at least one metal region and at least one electrically isolating region. A second security detection feature comprises metal-formed indicia. Where the metal regions of the repeating pattern serve to define the boundaries of the metal-formed indicia, the metal of such metal regions would need to have a sufficient thickness to render it visible. The benefit of such a device is that the first and second security detection features can be formed at the same time by depositing metal on a plastic thread or ribbon. However, such visible machine-readable repeating patterns can serve to interfere with the appearance of the visible and co-extensive metal-formed indicia, which serve as the public's means for establishing authenticity.
It is therefore an object of the present invention to provide a security thread that offers a generally invisible or transparent machine-readable security feature and also offers a public security feature.
It is also an object of the present invention to provide a security thread that offers an invisible or transparent machine-readable security feature that: has repeatable portions that extend the length of the thread; that does not interfere with the appearance of the visible security feature located on the thread; that facilitates high-speed machine reading; and that is not subject to obliteration.
It is yet a further object of the present invention to provide a security thread suitable for use with security documents, labels and any other document or means for identification used for purposes which make the verification of the authenticity of each specimen desirable at least once in its lifetime.